The present invention relates to the electrophotographic toner used in a printer, a facsimile machine and a copier, and to the image forming system using said toner. It relates particularly to color toner wnich does not require use of an oil or other anti-offset agent for the fusing device.
Recent improvements in computer processing capability have been accompanied by a quick expansion of the color printer market. Full-color image formation by electrophotography reproduces all colors, using the three colors of yellow, magenta and cyan, or four colors with black added thereto.
According to common practice, an electric latent image is formed on a photoconductor layer using a photoconductive substance. Then the latent image is developed by color toner, and the toner image is transferred onto a recording media, such as paper by an intermediate transfer unit. This is followed by the step of repeating the above-mentioned process several times for successive colors, thereby allowing multiple toner images of the different colors to be superimposed on the same recording media. In the final step, the toner image is fused onto the recording media in one operation by heating, pressure, thermal compression or solvent steam, thereby forming a full-color image.
In the color electrophotographic system requiring superimposition of multiple color toner images, toner fusing characteristics are very important. In the case of monochrome photography, fusing characteristics are evaluated only on the basis of the degree of sticking onto the recording media.
In the case of color photography, on the other hand, sticking onto the recording media alone is not sufficient to ensure an extensive color reproducibility. It is necessary to obtain gloss and transparency by sufficient melting (lower viscosity). This makes it necessary to use a Sharp Melt resin as the color toner, namely, a low molecular resin characterized by narrow distribution of molecular weight.
However, toner using such a resin is likely to have its internal condensation reduced at the time of heating and melting. Therefore, when it passes through the fusing device, it is more likely to stick to the fusing member. This will lead to vulnerability to an offset phenomenon.
To avoid this problem, in the fusing device commonly used for fusing color toner, silicone oil and other mold releasing agents are applied to the surface of the heating member of a roll or the like, thereby reducing adhesion between the molten toner and the heating material.
This method is very effective in preventing the offset phenomenon. However, the mechanism requires a complicated system of bigger size. Further, this method inherently has many problems, for example, reduction in handling properties due to deposition of oil on the recording media, and contamination of the system interior due to oil volatilization, not to mention the oil supplying time and labor and increased running costs. These factors make it difficult to achieve a smaller size, a lower price and a maintenance-free system.
To solve these problems, extensive studies have been made to develop a method of adding in the toner particles a wax which melts earlier than the resin without melting with the toner resin, or a mold releasing agent such as oil, instead of using the oil feeding device. Namely, when a mold releasing agent having a lower melting point than that of the resin is used, the mold releasing agent will melt before the resin melts and start flowing so as to ooze from the toner, and so a mold releasing layer is formed between the toner layer and fusing roller surface, thereby preventing the offset phenomenon.
If a highly viscous resin is used as in the case of a conventional monochrome toner, the internal coagulation is higher when the toner melts, and deposition onto the fusing member is smaller. This makes it possible to prevent the offset phenomenon with a small amount of oozing of the mold releasing agent. However, in the case of a color toner, heating and melting are accompanied by a greater reduction in the internal coagulation and a higher deposition on the fusing member. Not only that, a sufficient amount of mold releasing agent is required to ooze from the reduced internal coagulation. This makes it difficult to prevent the offset phenomenon.
If the amount of mold releasing agent is simply added, there will be an increase in the offset resisting effect, but the following problems occur if a great deal of mold releasing agent is added to the color toner:
(1) Resin and a mold releasing agent are incompatible with each other, so scattering occurs on the boundary, resulting in greater vulnerability to reduction in color tone and transparency.
(2) The mold releasing agent has the effect of a plasticizer on the resin, and decreases the resin strength and toner durability and toner durability.
(3) When the toner is kneaded, the mold releasing agent dissolves to decrease the effect of the kneading. This deteriorates the dispersibility of internal additives including the mold releasing agent.
Since a soft resin is used for color toner, the durability generally tends to be inferior to that of the conventional monochrome toner. This characteristic is made more marked by addition of the mold releasing agent. This gives rise to many problems, including deterioration of the toner flowing properties, and decrease in toner transportability, as well as sticking of toner to the blade in the non-magnetic component development method. Further, the mold releasing agent tends to cause filming on other members, such as the photoconductor, and this has been a problem to be solved.
These problems are caused not only by the amount of mold releasing agent added, but also by the physical state of the mold releasing agent in the toner. For example, if the mold releasing agent is poorly dispersed, the mold releasing agent is separated in the pulverization process in toner production and excessive exposure on the toner surface occurs.
As described above, the biggest technological issue for oil-free color toner is how to maintain durability, color development and transparency, and to improve the offset resistance.
In an effort to solve the foregoing problems, many proposals have been made to define the toner composition and the physical state of the mold releasing agent to be added in the toner.
For example, Japanese Official Patent Gazette No. 161153/1994  less than 1 greater than discloses a production method ensuring uniform distribution of mold releasing agents having a small diameter in the kneading and pulverization process. According to this method, the mold releasing agent in produced toner particles is said to exhibit a uniform dispersion in a spherical form having a dispersion diameter of 0.2 to 3.0 xcexcm, and/or in the form of a spindle with a major axis of 1.0 to 4.0 xcexcm. Without separation of the mold releasing agent, the agent is said to prevent filming on the photoconductor based thereon. The melting point of the mold releasing agent is preferred to be 90 to 180xc2x0 C., and more preferably to be 110 to 160xc2x0 C.
The Japanese Official Patent Gazette 69125/1998 ( less than 2 greater than ) also discloses the uniform dispersion of a mold releasing agent. It defines the dispersion of the mold releasing agent in terms of the ratio of the mold releasing agent contents in a fine powder toner and the entire toner. The disadvantage of the toner due to addition of said mold releasing agent is overcome by keeping the ratio within a specified range. This provides toner characterized by excellent offset resistance, according to this publication.
To overcome the difficulties of toner containing a mold releasing agent, many toner preparation methods based on a so-called wet process have been proposed. For example, Japanese Official Patent Gazette 197193/1993 ( less than 3 greater than ) discloses a capsule structure toner produced by a suspension polymerization method. This capsule structure toner makes it possible to reduce the amount of mold releasing agent exposed to the surface. This provides an easier solution to the problems, including deterioration of durability, filming properties and development characteristics, by addition of the mold releasing agent.
As described above, separation of the mold releasing agent or the adverse affect on other processes thereby can be minimized by uniform dispersion or encapsulation of the mold releasing agent.
According to the technique disclosed in Official Patent Gazette ( less than 1 greater than ), however, the melting point of the mold releasing agent is preferred to be90xc2x0 C. or more (most preferably 110xc2x0 C. or more). This is applicable to a monochrome system, but not applicable to a color electrophotographic system where a soft resin is suitable.
Furthermore, the technique disclosed in Official Patent Gazette ( less than 2 greater than ) is concerned with only the contents of the mold releasing agent, without any mention being made of dispersion diameter. The action of the mold releasing agent greatly depends on the state of dispersion, as well as the above-mentioned contents.
For example, the following has been confirmed: When the dispersion diameter is small, oozing of the mold releasing agent is not sufficient at the time of fusing even if the contents are sufficient, and so that offset phenomenon cannot be prevented. As described above, in the case of color toner, it is more difficult to prevent the offset phenomenon than in the case of a conventional monochrome toner. This is insufficient for an oil-free color toner.
The capsule structure toner disclosed in the Official Patent Gazette  less than 3 greater than  is characterized by easy delay of the mold releasing agent in the dispersion to the boundary. This results in a smaller contribution of the mold releasing agent to the resistance against the offset phenomenon. This requires a greater amount of mold releasing agent to be added.
There is a concern over possible reduction in color tone and transparency, as well as possible impact on durability. This may offset the advantages gained by encapsulation.
In the above-mentioned preparation method, the polymerizable monomer composition of the resin to be formed into the toner is restricted to styrene, a polymer thereof and a polymerizable monomer which allows polymerization of a solution typically exemplified by xcex1-methylene aliphatic monocarboxylic acid ester. This leads to the.problem that the polyester resin suited for use in conventional color toner is difficult to use. Further, this requires a special production plant and is more disadvantageous than the conventional kneading and pulverization method.
As is clear from the above, many technologies have been proposed regarding oil-free color toner, particularly the mold releasing agent to be added thereto, and the physical state thereof. In spite of such efforts, a color electrophotographic toner featuring excellent durability, gloss, color development and transparency, as well as a sufficient offset resistance, has not yet been obtained.
In view of the above, the main object of the present invention is to provide a new electrophotographic toner which does not require oil to be fed to a fusing system and which is suited to small-sized and maintenance free copiers and printers.
Another object of the present invention is to provide a color electrophotographic toner wherein consideration is given to the physical state of the resin as a toner constituent and the mold releasing agent added to the toner, thereby ensuring excellent melting and fusing properties, superb gloss, color development and transparency of the image, and a surpassing durability and offset resistance.
In accordance with the present invention, the electrophotographic toner used to develop an electrostatic latent image formed on a photoconductor based on image information toner comprises a colored powder formed by a mold releasing agent dispersed and pulverized in two or more resin mixtures having a different degree of crosslinking, and the overall occupancy rate of the mold releasing agent present as a domain in one particle of said toner is 1 to 10%, and the average dispersion diameter of the mold releasing agent is 10 to 50% of the toner particle diameter.
Namely, the present invention uses two or more types of resin having a different degree of crosslinking, and defines the physical state of the mold releasing agent in the toner. This provides electrophotographic toner characterized by sufficient melting and fusing properties. Further, this toner provides an image featuring excellent gloss, color development, transparency, durability, and resistance against the offset phenomenon. Particularly, the toner according to the present invention is outstanding as a color electrophotographic toner.